The role of nutrition on the development of the endocrine pancreas was studied in a rat model obtained by maternal food restriction. A 50 % food restriction was applied to female rats from day 15 of pregnancy and resulted in intrauterine growth-retardation (IUGR) in the offspring. At day 1 postnatal, beta-cell mass was significantly decreased in IUGR pups as compared to controls (0.70 ± 0.06 vs 1.07 ± 0.06 mg, p < 0.0001), as well as insulin content. This change in beta-cell mass can be attributed to a reduced number of islets, since the density of insulin-positive aggregates in pancreatic sections of IUGR rats was 20 % lower than in controls. Proliferative capacity of beta cells, as measured by 5-bromo-2-deoxyuridine (BrdU) labelling index, was not altered in growth-retarded animals. Body as well as pancreatic weight were fully recovered in IUGR pups after 21 days of normal feeding by control mothers. However, these animals retained a 25 % decrease in insulin content, 40 % decrease in beta-cell mass (1.58 ± 0.18 vs 2.78 ± 0.42 mg, p < 0.001) and a strong reduction in the density of insulin positive aggregates per cm², as compared to controls, suggesting that the total islet number was likely to be reduced. Beta-cell proliferative capacity remained normal. In conclusion, in utero undernutrition in rats does not impede postnatal growth but durably impairs beta-cell development. Impairment of beta-cell differentiation might be suggested. [Diabetologia (1997) 40: 1231–1234]